The following account of the prior art relates to one of the areas of application of the present invention, hearing aids.
In feedback cancellation systems in hearing aids, it is desirable that the output signal (i.e. receiver signal) u(n) is uncorrelated with the target input signal x(n). In this case, the algorithm used for updating the parameters of the feedback cancellation filter is typically operating under the theoretical conditions for which it is derived, and the performance of the feedback cancellation system can be good. However, unfortunately in hearing aid applications the output and input signals are typically not uncorrelated, since the output signal is in fact a delayed (and processed) version of the input signal; consequently, autocorrelation in the input signal leads to correlation between the output signal and the input signal. If correlation exists between these two signals, the feedback cancellation filter will not only reduce the effect of feedback, but also remove components of the input signal, leading to signal distortions and a potential loss in intelligibility (in the case that the input signal is speech) and sound quality (in the case of audio input signals).
The traditional way of getting an output signal which is uncorrelated with the input signal is by using probe noise, where a signal-dependent noise source, uncorrelated with the input signal, is added to the output signal. Although probe noise techniques in principle can reduce the autocorrelation problem, there are a number of disadvantages that make these techniques less than ideal. First, the probe noise must be inserted such that, ideally, it is completely masked by the original output signal, and thus inaudible for the listener. This, in turn, means that the probe noise level is very low compared to the input signal, leading to a low “probe noise-to-interference ratio”, where “interference” in this context is the target signal impinging on the microphone, e.g. speech/audio, etc. The consequence of this is a larger variance on the feedback path estimate or/and a long adaptation time. Furthermore, with probe noise techniques the adaptive feedback cancellation filter coefficients are typically estimated based on the probe noise alone, but ignores the potentially useful signal components of the original output signal leading to unnecessary poor working conditions for the adaptive system.
WO 2007/006658 A1 describes a system and method for synthesizing an audio input signal of a hearing device. The system comprises a filter unit for removing a selected frequency band, a synthesizer unit for synthesizing the selected frequency band based on the filtered signal thereby generating a synthesized signal, a combiner unit for combining the filtered signal and the synthesized signal to generate a combined signal.